Chapter 8: Introduction to Atomic Spectrometry Read: pp. 215 228 Problems: 2,4,5,6,9 Why choose atomic spectrometry? Three major types of spectrometric methods for identifying elements present in matter: optical spectrometry (Chap. 8, 9, 10) mass spectrometry (Chap. 11) x-ray spectrometry (Chap. 12) In optical spectrometry, the elements present in a sample are converted to gaseous atoms or elementary ions and then analyzed by optical methods.
403.08 Mn 403.31 403.45 Wavelength (nm) Notice high resolution! Excellent series of methods for determining elemental composition in environmental samples, foods and drinks, potable water, biological fluids, and materials.
Atomic Spectra Outer shell or valence electrons are promoted to unoccupied atomic orbitals by incident radiation. E = hν = hc/λ Small energy differences between the different transitions so high resolution instruments are needed. Na Transitions are observed only between certain energy states. Figure 8-1a
Na Mg + Figure 8-1
Figures 8-2 and 8-3 Mg
Chemical Problem The first excited state of Mg is reached by absorption of 457.1 nm light. Calculate the energy difference (kj/mol) between the ground and excited states. E = hν = hc/λ E = (6.62 x 10-34 J s)(3.00 x 10 8 m/s) (457.1 nm)(1.00 x 10-9 m/nm) = 4.34 x 10-19 J/photon (4.34 x 10-19 J/photon)(6.02 x 10 23 photons/mol) = 2.62 x 10 5 J/mol (2.62 x 10 5 J/mol) (1 kj/1000 J) = 262 kj/mol
Atomic Line Widths Sources of Line Broadening 1. Uncertainty effect 2. Doppler effect 3. Pressure or collisional effects 4. Electric and magnetic field effects Figure 8-6
Uncertainty Effect Spectral lines have finite widths because lifetimes of one or both transition states are finite, which leads to uncertainties in transition times. ν t 1 To know ν with high accuracy, then time of measurement, t, must be very long! Lifetime of ground state is long, but lifetime of excited state is short, 10-8 s. Line widths due to uncertainty broadening, sometimes called natural line widths, are on the order of 10-4 Å.
Doppler Broadening λ/λ o = v/c v = velocity of moving atom Encounter wave crests more frequently Encounter wave crests less frequently Maximum None Mixed Wavelength of radiation emitted or absorbed by rapidly moving atom decreases if motion is toward the detector and increases if motion is away from the detector. Broadening on the order of 10-2 to 10-1 Å.
Pressure Broadening Broadening arises from collisions of the absorbing or emitting species with other atoms or ions in the heated medium. Collisions cause small changes in the ground state energy levels and, thus, a range of absorbed or emitted wavelengths. Broadening on the order of 10-1 Å.
Temperature Effects Temperature influences velocity and, thus, the extent of Doppler broadening and pressure broadening. Temperature also influences the number of atoms in the ground (N 0 ) and excited (N j ) states N N j 0 = g g j 0 E j exp k T where g 0 and g j are statistical factors and E j is the energy difference between states. So temperature should be maintained constant, as much as possible.
Least effect on atomic absorbance and fluorescence measurements, because fraction of atoms in ground state is large. Greatest effect on atomic emission measurements, because fraction in excited state is small.
Sample Introduction Methods Goal is to produce neutral atoms or simple ions in the gas phase Sample may be gas, liquid/solution, slurry, or solid Sample introduction may be continuous or discrete Great challenge generally limits accuracy, precision, and detection limits of atomic spectrometric methods
Steps in Sample Introduction Process Several methods for introduction of liquid samples, main one is pneumatic nebulization: Figure 8-10
Pneumatic Nebulizer Designs Concentric Tube Cross-Flow Figure 8-11
Graphite Tube Furnace Sample Liquid or solid samples Smaller sample volumes More efficient atomization Lower detection limits Poorer precision Slow requires heat cycles to dry, ash, atomize hν Figure 9-6a